1. Field of the Invention
The present invention relates to a cascade electromagnetic pulse protection circuit for high frequency application, which can inhibit the fast or slow overvoltage surge induced by EMP (Electromagnetic Pulse).
2. Description of the Related Art
Electromagnetic weapons may be classified into the NEMP (Nuclear Electromagnetic Pulse) bomb, the HEMP (High-altitude Electromagnetic Pulse) bomb, the PEMP (Portable Electromagnetic Pulse) weapon. The NEMP bomb has an affected area with a radius of hundreds of kilometers. As a nuclear explosion can damage so vast an area and will cause so intense an international reaction, it is very unlikely to be directly used in the future war. Besides, the NEMP bomb may also damage our own communication systems and monitoring systems. The EMP warhead carried by a long-range missile has a medium affected area with a radius of tens of kilometers. The PEMP weapon, which is a small-size bomb carried by a human being or a small-size carrier, and has an affected area with a radius of less than one kilometer. In the long run, EMP bombs must be one of the weapons used in war.
The explosion of an EMP weapon will generate a pulse of a very high strength electric field characterized in that the electric field strength rises so quickly that the maximum strength is reached within 10 nanoseconds, and that the electric field strength generated by the EMP weapon explosion is higher that that of an LEMP (Lightning Electromagnetic Pulse) and is between 50 and 100 KV/m. The fast rising of so intense an electric field strength will induce a current as great as thousands of amperes in an antenna or a longer cable. Because of so fast a strength rising rate, so high a voltage and so great a current, general lightning arresters are unlikely to protect electronic devices against the damage caused by an EMP weapon explosion.
When an EMP rushes into a communication/information device, it may induce a voltage as high as from 10 to 150 kV. If there is none protection circuit, the front end electronic elements are unlikely to tolerate such a high voltage. Currently, communication electronic devices generally have so-called lightning arresting devices, which are usually low-speed LEMP protection elements formed of spark gap switches or zinc oxide elements. As the LEMP protection element operates slowly, it has a long response time and a higher clamping voltage.
For current LEMP protection elements, the EMP inhibition effect is determined by the response time. As the spark gap switches or zinc oxide elements have a longer response time, they will not inhibit EMP until the surge voltage has been very high. Therefore, the current LEMP protection elements cannot protect an electronic device against a fast-rising EMP electric field.
IC (Integrated Circuit) generally has a high-speed ESD (Electrostatic Discharge) protection element (usually TVS (Transient Voltage Suppressor), DIAC (Diode for Alternating Current), MOV (Metal Oxide Varistor), etc.) at the front end. The high-speed ESD protection element can respond very fast and can tolerate a voltage as high as 8000V. However, the high-speed ESD protection element is not a high current tolerant element. The electromagnetic pulse of a lightning or an EMP bomb has a greater pulse width. A longtime overvoltage induces a current effect and generates a large amount of heat, which will firstly burns out the high-speed ESD protection element and then burns out the internal structures of IC. Therefore, a high-speed ESD protection element can only withstand electrostatic discharge but cannot tolerate high-energy EMP attacks.
Generally, a communication device has a lightning arresting device (LEMP protection element), and an IC has an ESD protection element at the front end thereof. However, even parallel connecting an LEMP protection element and an ESD protection element cannot yet solve the abovementioned problem. Current will firstly flow through the high-speed ESD protection element, which turns on fast but cannot tolerate high current, until it burns out. Then, the voltage rises again. The front end IC maybe has burned out before the LEMP protection element starts to operate. Besides, as all high-speed protection elements are capacitive elements, they will result in the insertion loss of communication/information devices and affect communication distance.
A Taiwan patent No. 588888, which was filed by the Inventors on Sep. 24, 2002, disclosed a method to solve the above-mentioned problem, wherein a rear stage ESD protection element is triggered firstly, and the potential is established on a cascaded resistor, and then a front stage high-power LEMP protection element is triggered reversely. Thus, the charge following can be drained out, and the entire system is protected. Besides, the insertion loss of input signal is also compensated. However, the conventional technology still has the problem of insertion capacitance, which is shunt to the ground and likely to cause high-frequency loss. In applications that the frequency is higher than 1 GHz, if the small-capacitive MOV or TVS elements are adopted; thus, the current draining capacity is also smaller, and the EMP impact on the performance of electronic devices cannot be effectively reduced. For low-frequency applications, the conventional technology still has other problems to be solved. For an electric field strength higher than 300 KV/m, the conventional technology still has room to improve.